In generally, sealing property between the sealing surfaces of the seat ring and closure member is a significant factor to affect leakage of the valve when it is in its fully closed position.
The valve in which the seat ring is made of polymer, known commonly as soft seated valve, has excellent sealing property, and the galling does not occur between the sealing surfaces of its seat ring and closure member so easy as metal seated valve does because of low friction coefficient of the polymer. The valve has long enough lifetime in general service, and is widely used.
For the fluid flowing through the valve at a higher temperature or carrying abrasive particles and/or powders, high temperature and wear resistant metal seated valve with hard facing is usually recommended to be first selected. Hard alloy and the like are normally coated, plated or welded on the surfaces of the seat ring and closure member when the metal seated valve are designed and manufactured. It is well known that the trademarks of mating coating material coated on the surfaces are different from each other generally, and their hardened hardness also varies in order to protect the sealing surface of the seat ring and the surface of the closure member against mutual abrasive wear or galling. The mating surface has to be machined precisely, too, and grinded with each other if necessary so as to increase their mutual sealing property. Therefore, not only the coating material should be resistance to high temperature, abrasion and corrosion, but it is also possessed of strong bond strength with its metallic matrix, furthermore it has low friction coefficient between the coated surfaces for lower operative torque or thrust of the valve.
In recent years, valve manufacturers in the world have diligently been searching for the materials possessing a low-friction coefficient, corrosion resistance, high hardened hardness, abrasive wear resistance and an ability being not apt to be scuffed, such as known from Node, et al., U.S. Pat. No. 5,108,813. Even so, abrasive wear or galling occurs frequently between the sealing surfaces of the seat ring and closure member of metal seated valve in actual service.
Frictional wear is a microscopic dynamic process occurred in material surface, and a complex process relating to behaviors of many subjects including mechanics, materials science, physics, chemistry and heat transfer science. According to analysis from the standpoint of physics and chemistry, wear occurs in the surfaces of two objects in relative sliding motion, and in a very thin layer of working surface. An important characteristic in the course of wear is that mechanical energy changes into heat energy, and the heating or cooling proceeds at very high velocity.
A lot of tests have shown that the abrasive wear is caused by hardness difference and galling by frictional heat. The real area of contact between mating surfaces of two bodies in relative sliding contact is far less than apparent area of contact, even though they are machined finely. The reason is that any surface has waviness and roughness whatever machining is made, and is seen microscopically as a series of asperities presenting serrated peaks, rather than the flat surface seen macroscopically, therefore the real interface of junctions between two relative sliding surfaces is only at the top of some peaks projected outwards from the surface. The softer peaks having the real interface will be sheared or microploughed by the harder peaks facing them during sliding contact relative to each other with the result that a wear fragment transfers from one surface to the other, and loss of material occurs. Meanwhile instantaneous high temperature caused by compression deformation or break at the interface will heat up the top of the peaks having small volume and tiny thermal capacity to cause their temperature to rise sharply and material of the top to become hardened, softened or phase transition. The instantaneous high temperature has the opportunity of causing the material of the interface to be risen up to melted level if the two surfaces slide quickly and repeatedly over a longer distance against each other under a bigger load, and the environment for dissipating heat around them is not so good, as a result abrasive wear or galling will occur at interface of the junctions between two surfaces.
It is also known that the abrasive wear or galling depends not only on a load exerted on sliding contact surfaces and relative sliding velocity between the two sliding surfaces, but on a distance that a smaller object slides over on a larger object, according to friction law that frictional force is proportion to contact load normal to the surfaces published by Mr. Amontons and Mr. Couloms, and the equation that material wear is proportional to load exerted on the surfaces and a distance sliding over relative to each other given by Mr. Archard.
Mr. Laitinen, et al. introduced a new parameter according to Mr. Archard's equation------lineal contact length (that is the overall sliding distance) affecting galling between the sealing surfaces of the seat ring and valve plug member in their WO patent publication No. 02/33299. Assuming static conditions and normal materials, the effect of the lineal contact length on the galling conditions can be determined from equation:Galling factor=PVLn 
In this equation, P is the surface load exerted on the sealing surfaces of the seat ring and plug member, V is the sliding velocity during their relative motion, L is the lineal contact length that is a distance slid over on the sealing surface of the seat ring by a given stationary point in the surface of the plug member from this point contacting a leading edge of the sealing surface of the seat ring to it leaving its trailing edge, and n is an exponent having a value greater than 1.
When the seat ring is sliding on the surface of the plug member relatively, the lineal contact length L along its periphery varies in sliding direction of the plug member, therefore this value has a significant effect on the distribution of the frictional heat on the surface of the plug member, particular on the sealing surface of the seat ring.
The lineal contact length must be as short as possible in order to avoid or mitigate the abrasive wear and galling caused by frictional heat between the sealing surfaces of the seat ring and plug member. The embodiment disclosed by Mr. Laitinen, et al in WO patent publication No. 02/33299 describes that the recesses, grooves or concavities slightly staggered relative to each other in the direction of movement of the plug member are arranged on the sealing surfaces of the seat ring (if possible, including the surface of the plug member) in a dense pattern but spaced apart from each other, so that a continuous contact with the valve plug member in the lineal contact length is interrupted and divided into several portions without causing leakage passageways when the valve is in its fully closed position. These recesses, grooves or concavities may provide cooling spaces of dissipating the instantaneous frictional heat, not letting the temperature at the top of the peaks in the frictional surfaces rise over high, thereby reduce the possibility of the abrasive wear and galling between the sealing surfaces of the seat ring and plug member. But the effect is unsatisfactory either because the recesses, grooves or concavities are difficult to machine or because the instantaneous high temperature accumulated at the tops cannot be timely dissipated in some cases.
The fluid transported in pipeline becomes severer than ever with the great headway and rapid development of all kinds of process industries, and operation temperature, opening or closing speed, operational frequency and sealing property requirements of the valve are also higher and higher. Abrasive wear or galling occurs frequently between the sealing surfaces of the seat ring and closure member of metal seated valve due to generation of a great deal of frictional heat or difficulty in dissipating it when the valve is used in the higher temperature and the fluid carrying harder and harder particles or powders, even though it is operated in the range of suitable temperature. The sealing surface of the seat ring made of polymer will be carbonized by instantaneous high temperature caused by frictional heat in quick opening or closing and frequent operation, with the result that movement of the valve closure member becomes sluggish or is not agile. How to find a solution to the abrasive wear or galling due to instantaneous frictional heat begins to become a subject that valve manufacturers devote much time to their research.